Acanthostega, a Stem Tetrapod – Life History Revealed by Synchrotron microtomography

Australia: The Land Where Time Began

Acanthostega, a Stem Tetrapod – Life History Revealed by Synchrotron microtomography

According to Sanchez et al. the transition from fish to tetrapod was arguably the most radical shifts in vertebrate history. For most aspects of these events (Clack, 2012; Niedźwiedzki et al., 2010; Shubin, Daeschler & Jenkins, 2006; Friedman, Coates & Anderson, 2007; Boisvert, Mark-Kurik & Ahlberg, 2008) data have been rapidly accumulating, though for the earliest tetrapods the life histories have remained completely unknown, with a major gap in understanding of these organisms as living animals. The unspoken assumption that the largest known tetrapod fossils from the Devonian represent adult individuals is symptomatic of this problem. In this paper Sanchez et al. present the first, as far as they know, life history of a tetrapod dating to the Devonian, from the deposit of Stensiö Bjerg in East Greenland (Astin et al., 2010; Blom et al., 2007), which contained evidence of mass-death of Acanthostega. Sanchez et al. have shown that even the largest individuals in this deposit were juveniles by the use of propagation phase-contrast synchrotron microtomography (PPC-SRμCT) (Sanchez et al., 2012) to visualise the histology of humeri (upper arm bones) in order to infer their growth histories. It was found that juvenile Acanthostega underwent a long early juvenile stage during which their limb bones were not ossified and individuals grew to almost final size, which was followed by a late juvenile stage that lasted at least 6 years, in which they grew slowly and had ossified limbs. Juveniles are suggested by the late onset of ossification of limbs to have been exclusively aquatic, and it is suggested by the predominance of juveniles in the sample that at least at certain times distributions of juveniles and adults were segregated. The possibility of sexual dimorphism, adaptive strategies or composition-related size variation is suggested by the absolute size at which ossification of limbs began differs greatly between individuals.

There has been much speculation on the transition from water to land and the role the life cycle had in this process. E.g., it has been suggested that the earliest tetrapods returned to ephemeral pools to reproduce, and that when the larvae needed to travel over land or through water that was extremely shallow to relocate from the ponds that were drying up to water bodies that were more permanent was the selective pressure to evolve towards terrestriality (Warburton & Denman, 1961). The fossil record of tetrapods from the Devonian has, however, been dominated by specimens that were rare and incomplete that are often recovered from localities that are poorly constrained such as scree slopes (Blom, Clack & Ahlberg, 2005), has until recently yielded virtually no data on the life history of these tetrapods.

The deposit in the Britta Dal Formation on Stensiö Bjerg, East Greenland (Blom, Clack & Ahlberg, 2005), the site of the Acanthostega mass death, is the only known tetrapod locality with potential for revealing information on the life history, which dates to the Devonian. This locality, which is comprised of a small in situ micaceous silty sandstone body and scree which is immediately associated (Blom, Clack & Ahlberg, 2005), has to date yielded more than 200 skeletal elements. Among the 14 skulls recovered from this site that were associated with skeletons that were partially articulated, which were complete enough to measure (Clack, 2002), and there were several more which could be identified as individuals; Sanchez et al. estimating that there were at least 20 animals represented, though there were almost certainly more present. There were only a few isolated bones that represented other vertebrates. The individual Acanthostega that were recovered from this site appeared to have died together, probably during a drought that followed a sheet flood event (Astin et al., 2010); therefore, they represent a single time-point sample from a population of this stem tetrapod. Sanchez et al. used the non-destructive imaging technique PPC-SRμCT (Sanchez et al., 2012), performed at beamline ID19 of the European Synchrotron Radiation Facility (ESRF) to undertake historical investigations of the 4 humeri that had been collected from the locality (Natural History Museum of Denmark MGUH 29019, MGUH 29020, NHMD 74756; University Museum of Zoology Cambridge UMZC T.1295 (Coates, 1996), to recover data that illuminate the life history of Acanthostega. All of these are humeri of Acanthostega are the only ones known to date. The other humeri that have been recovered are isolated bones. These Acanthostega humeri are of 2 distinct size categories – large (NHMD 74756, MGUH 29020) and small (MGUH 29019. UMZC T.1296). Sanchez et al. found no correlation between size and the degree to which ossification had progressed, which is consistent with previous observations (Callier, Clack & Ahlberg, 2009): specimens NHMD 74756 and UMZC T.1295 are weakly ossified while specimens MGUH 29019 and MGUH 29020 are strongly ossified.

An extensive spongiosa surrounded by a thin compact cortex is exhibited by all humeri. This arrangement is similar to that of the humerus of the lobe finned fish Eusthenopteron (Sanchez, Tafforeau & Ahlberg, 2014), which a member of the tetrapod stem group that is less crownward (Coates, Ruta & Friedman, 2008). In the metaphyseal region, which is close to the articular extremities) there are remnants of calcified cartilage, which show that the spongiosa formed by endochondral ossification as occurs in extant tetrapods (Francillon-Vieillot et al., 1990) and Eusthenopteron (Sanchez, Tafforeau & Ahlberg, 2014; Lauren et al., 2007). At the base of the epiphyses there are tubular structures resembling the marrow processes in the growth plate of the humerus of Eusthenopteron (Sanchez, Tafforeau & Ahlberg, 2014).

There is a dense arrangement of radial vascular canals similar to those of juvenile Eusthenopteron in the midshaft cortex of all Acanthostega humeri. The radial canals are connected to a basal mesh of canals that are parallel to the surface. The largest specimen, MGUH 29020 has radial canals that vary in diameter between different parts of the area that is being scanned, which probably reflects local blood supply needs. All appear to retain areas of primary internal cortical surface, though in 3 of the humeri there is evidence in the cortex of patchy basal erosion. Between the endosteal bone and the cortex there are clusters of large aligned globular cell lacunae that ca be identified as chondrocyte lacunae by comparing them with juvenile Eusthenopteron, which have similar lacunae between the cortical bone and remnants of calcified cartilage that have not been resorbed (Sanchez, Tafforeau & Ahlberg, 2014). The perichondral surface of the original cartilaginous humerus is marked by these many alignments of chondrocyte lacunae at midshaft. It is implied by this that, as limb bone growth originates at the midshaft, the cartilaginous rod was very large relative to the final size of the bone that was observed, and that the growth of cortical bone conversely only made a modest contribution to its final size. I.e., the Acanthostega individuals grew to almost full observed size before the humeri began to ossify.

Lines of arrested growth (LAGs) are present in the cortical bone and this allows the inference of the number of years that were occupied by the deposition of this tissue, based on the assumption that the deposit between 2 LAGs represents an annual cycle, which is the case in most extant tetrapods (Castanet, Francillon-Vieillot & de Ricqlès, 2003; Padian, 2012). It has been revealed by observations of the 4 humeri that there are a maximum number of 6 LAGs in MGUH 29020, 4 in NHMD 74756 and UMZC T.1295, and 3 in MGUH 29019. All observations were made in areas that were at least partially covered by matrix and therefore are not likely to have been affected by external erosion. These LAG patterns are regular and do not show tightening – i.e., there was no growth rate deceleration – as would be expected at sexual maturity in adult tetrapods (Padian, 2012; Sanchez, 2008; Castanet et al., 1993). It is suggested by this that the 4 specimens of Acanthostega were juveniles at the time of their death, if it is assumed that their humeri had begun to ossify prior to the onset of sexual maturity, as they do in all tetrapods that are known (Fröbisch, 2008; Witzmann, 2006; Schoch, 2004 and in Eusthenopteron (Sanchez, Tafforeau & Ahlberg, 2014). It is suggested by Sanchez et al., therefore, that the juvenile stage in Acanthostega must have lasted for at least 6 years. They also suggested that it probably lasted a good deal longer, as the cartilaginous humerus grew to almost full size prior to the deposition of cortical bone, and therefore the recording of annual growth increments, even began. Acanthostega is not the only member of the tetrapod stem group to display late onset of ossification. A large spongiosa and a cortex with no internal resorption is exhibited by juvenile Eusthenopteron, which shows that the original cartilaginous rod was about ⅔ of the adult size of spongiosa, and it is presumed to have formed over several years (Sanchez, Tafforeau & Ahlberg, 2014). It is difficult to say how this relates to final adult size in Acanthostega, though it is suggested by the slow growth rate of the juvenile Acanthostega that final adult size may not have been much greater than the largest individuals that have been recorded from the mass death deposit.

Sanchez et al. suggest the complete lack of correlation between size and degree of ossification could be a reflection of some form of individual variation, such as the variation of size related to competition that is observed in certain extant tetrapods (Peacor & Pfister, 2006), adaptive strategies or sexual dimorphism (Badyaev, 2002). Some individuals, represented by MGUH 29019 and UMZC T.1295, began ossifying their humeri, and presumably approach sexual maturity, while at a much smaller size than others, which were represented by MGUH 29020 and NHMD 74756, under these interpretations. It was not possible because of the small size of the sample to Determine whether the apparently discrete size classes are a reflection of a real bimodal size distribution, or whether they are simply the result of a continuous size variation that was sampled randomly. The construction of an ontogenetic sequence from smallest to largest humerus was invalidated categorically, however, by the observed combination of sizes and ossification states.

New light was shed on several aspects of palaeobiology and life history of Acanthostega by the synchrotron virtual histological data derived from the humeri. As shown by the LAGs, Acanthostega had a prolonged juvenile stage of no less than 6 years, though more probably at least 10 years, given that it grew to almost full recorded size before the onset of cortical bone ossification. According to Sanchez et al. this aligns it with a range of sarcopterygian fishes and tetrapods that included Neoceratodus (15-20 years (Kind, 2002)), Eusthenopteron (adulthood at 11 years (Sanchez, Tafforeau & Ahlberg, 2014), Discosauricus (10 years (Sanchez et al., 2008)) and Andrias (larval period of 4-5 years and 10 years to adulthood (Sparreboom, 2014)), which suggests a long juvenile stage could be the primitive condition for tetrapods. It is implied by the late onset of ossification in Acanthostega that the early juvenile stage was aquatic, as a cartilaginous humerus would not be suitable for locomotion on land; this also agrees with the presence of aquatic adaptations that include a large caudal fin and a well-developed gill skeleton in Acanthostega (Clack, 2012; Ahlberg & Milner, 1994), and contradicts the hypothesis that juveniles were terrestrial (Warburton & Denman, 1961) at least for this particular tetrapod.

It is suggested by the fact that all 4 of the humeri appear to be from juvenile individuals that the mass death assemblage is dominated by, and could possibly consist exclusively of, juveniles, a set of distinctly larger individuals is not included in the assemblage. The most fully ossified humeri of the assemblage were from specimens MGUH 29019 and MGUH 29020. MGUH 29019, which is the smallest humerus, is associated with a skull that was 12 cm long; MGUH 29020 is the largest humerus, was an isolated find from the scree, though it appears to represent one of the largest individuals in the assemblage (personal observation, J.A.C. and P.E.A.).

A context is provided for these observations by the palaeoenvironmental data from this locality. It forms part of a large ephemeral fluvial system in what is otherwise an arid topical landscape (Astin et al., 2010), which extends to the north for more than 200 km from the source water body that has not been preserved that must have been large and permanent as it was home to large lobe-finned fishes such as Eusthenodon and Holoptychius (Blom et al., 2007). It appears the Acanthostega individuals were flushed out into the fluvial system during a flood event, then an ensuing drought concentrated them is a shrinking pool that eventually dried out, which killed them (Astin et al., 2010). It is suggested by the almost complete absence of other taxa in the death assemblage that it is not a whole fauna that has been concentrated (as was the case in a near-contemporary mass death deposit from Canowindra, Australia, rather it may be a reflection of schooling behaviour in Acanthostega. Sanchez et al. concluded, therefore that Acanthostega had a long aquatic juvenile stage that was characterised, at least in certain times, by the formation of schools that included few if any adults.

The type of life history information that is provided by the humeri is dependent only on the preservation of the actual bone and it potentially can be matched in a wide range of stem tetrapods, whereas the unique palaeoenvironmental and population related data that are provided by the Acanthostega mass death deposit depend on the context of that particular locality. In principle, a single limb bone can provide decisive answers to questions concerning the length of the juvenile stage and at what age the onset of ossification occurs, which in turn help in constraining the palaeobiological hypothesis. Sanchez et al. are undertaking a systematic PPC-SRμCT survey of the limb histology of stem tetrapods with this aim. Acanthostega provides, for now, a first glimpse of the life history of a tetrapod from the Devonian.

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